Field of the Invention
The present invention relates to the making of irradiated regions in materials such as semiconductor bodies with nuclear particles.
There are many situations where it has been desired to form an irradiated region of a precise thickness, dosage and dosage gradient in a material with nuclear particles of molecular weight of at least one (1). This has not, however, been generally possible in the past without great difficulty.
Nuclear particles of molecular weight of at least one (1) cause localized damage in materials on irradiation. The irradiation damage or defect generation by such nuclear radiation is concentrated in a relatively narrow region near the end of the particle penetration into the material. The irradiated regions thus formed have been dependent upon the energy and energy distribution of the available radiation sources. Most such radiation sources are essentially monoenergetic beams produced by Van de Graaff accelerators. Irradiated regions produced by such monoenergetic sources are gaussian shaped with a very narrow half-width. Thus, if an irradiated region from nuclear radiation with a molecular weight of at least one (1) was needed in a material, the material would have to be irradiated successively with radiation sources of different energies and different dosages. This procedure was time consuming and in many instances impossible.
The thickness of such irradiated regions has been extended without successive irradiations by pivotally moving the material in an oscillatory manner, but even with this technique, the thickness of the irradiated region was typically less than 5 um. Scattering foils have been placed between the beam generator and irradiated material, but these foils were necessarily of uniform thickness to provide the desired scattering of the beam and substantially uniform dosage over a large area of the material. It was not proposed to shape a material of low scattering properties to modify and modulate the beam energy to tailor an irradiated region of desired thickness, dosage and dosage gradient.
The present invention particularly provides an improvement on the invention claimed in U.S. Pat. No. 4,056,408, as well as the other inventions claimed in applications and patents referenced there, all of which are assigned to the assignee of the present application. The invention claimed in U.S. Pat. No. 4,056,408 involves reducing the switching time of semiconductor devices by precisely locating an irradiated region formed by nuclear irradiation in the device adjacent a blocking PN junction of the device. Although the invention there described has particular value, its applications and benefits have been limited by the energies and energy distributions of available radiation sources.
The present invention has particular application in making semiconductor devices having one or more blocking PN junctions in their structure. Such semiconductor devices include thyristors, transistors, diodes, diacs, triacs, reverse switching rectifiers and reverse conducting thyristors among others. Such semiconductor devices are required in their operation to change from a high conduction mode to a low conduction, high blocking mode rapidly. This change, which has typically been defined as switching time or reverse recovery time or reverse recovery charge has been limited to the diffusion and recombination lifetimes of the carriers in the device. In turn, the applications of such devices, and particulalry high power devices, have been correspondingly limited.
As explaind in U.S. Pat. No. 4,056,408, applicant and his colleagues have sought to decrease the swtiching time and reverse recovery time of semiconductor devices by various irradiation methods. These have involved, however, to one degree or another a trade-off with other electrical characteristics and most notably forward voltage drop of the semiconductor device. That is, if a very low turn-off time or reverse recovery time is desired, a higher forward voltage drop had to be tolerated in the device. The same is true if a low reverse recovery charge is desired.
The present invention substantially reduces the impact of this trade-off in tailoring the electrical characteristics of semiconductor devices by irradiation. More broadly, the invention also provides a means for precision forming of irradiated regions in semiconductor bodies and materials generally with accurate control over the thickness of the irradiated region, and its dosage and dosage gradient. The present invention also permits forming an accurately dimensioned and dosage contoured irradiated region internally of a material a desired distance from the selected surface of the material.